1. Field of the Invention
The present invention relates to a jet noise modeling method of modeling jet noises generated by a jet stream jetted by a jet engine, a jet noise analyzing method, and an aircraft designing method using the jet noise modeling method and the jet noise analyzing method.
2. Description of the Related Art
An aircraft design is required to include means for reducing jet noises generated by a jet stream jetted by an engine and uses the shielding effect of an airframe on isolating jet noises. It is necessary to analyze a sound field of jet noises to examine the noise shielding effect of an airframe. A jet stream is analyzed by unsteady Navier-Stokes analysis (NS analysis), such as large eddy simulation (LES) or direct Navier-Stokes (DNS) analysis. The condition of the sound field of jet noises is determined on the basis of a pressure variation estimated from the results of analysis. Unsteady NS analysis takes extremely much time. It is very difficult for the ability of a current computer to achieve the analysis of a jet stream jetted by an actual jet engine.
An aerodynamic sound source search system disclosed in JP-A 2005-3368 determines a sound source distribution of aerodynamic sounds generated around an object placed in a field of flow. This aerodynamic sound source search system calculates the distribution of velocity vectors in a predetermined finite computational region, and calculates a sound source distribution around the object on the basis of the distribution of velocity vectors, taking into account the influence of vortices outside the computational region. This aerodynamic sound source search system sets a predetermined finite computational region as a range for calculating velocity vectors in the field of flow, converts the influence of vortices outside the computational region into the influence of vortices in the computational region, and determines the distribution of dipole sound sources by using an expression developed by combining the Howe's vortex expression and a compact Green function suitable for the shape of the object. Although this aerodynamic sound source search system can determine the distribution of sound sources in the field of flow, this aerodynamic sound source search system cannot be applied to the analysis of a sound field of jet noises.
A noise environment evaluating system disclosed in JP-A 6-4512 is used for analyzing the levels and distribution of noises generated by a plurality of noise sources. This noise environment evaluating system enters data on the arrangement of buildings including walls, and noise sources, produces a sound ray model of sound propagation passages from the data on the arrangement of buildings and the noise sources, determines sound pressure levels at observation points on the sound propagation passages by using an expression expressing sound attenuation in distance, and synthesizes the thus determined sound pressure levels. This noise environment evaluating system can simply evaluate the levels and distribution of the current noises by entering data on the arrangement of buildings, and noise sources. Thus this noise environment evaluating system can simulate the changes of a noise environment resulting from change in planning conditions without restraint. However, this noise environment evaluating system cannot be applied to the analysis of a sound field of jet noises.
A noise level predicting method is disclosed in JP-A 1-227023. This noise level predicting method predicts a noise level on the basis of the sound power level of a sound source and distance from the sound source. This noise level predicting method calculates the mean sound power level of the sound power levels of a plurality of element planes defined by circumferentially dividing a semispherical plane having its center at the sound source, and calculates the respective ratios of the sound power levels of the element planes to the mean sound power level to use the same as noise directivities for correcting the noise levels. This noise level predicting method cannot be applied to the analysis of a sound field of jet noises.